Send ScalaNativeWorker logging to stderr (#4418)

Some log levels for the ScalaNativeWorker were writing to stdout instead
of stderr, this switches those over to stderr.
Closes #4409

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Li Haoyi <haoyi.sg@gmail.com>

scalanativelib/worker/0.5/src/mill/scalanativelib/worker/ScalaNativeWorkerImpl.scala

@@ -1,7 +1,7 @@
 package mill.scalanativelib.worker
 
 import java.io.File
-import java.lang.System.{err, out}
+import java.lang.System.err
 
 import mill.scalanativelib.worker.api._
 import scala.scalanative.util.Scope
@@ -29,9 +29,9 @@ class ScalaNativeWorkerImpl extends mill.scalanativelib.worker.api.ScalaNativeWo
   def logger(level: NativeLogLevel): Logger =
     Logger(
       traceFn = msg => if (level.value >= NativeLogLevel.Trace.value) err.println(s"[trace] $msg"),
-      debugFn = msg => if (level.value >= NativeLogLevel.Debug.value) out.println(s"[debug] $msg"),
-      infoFn = msg => if (level.value >= NativeLogLevel.Info.value) out.println(s"[info] $msg"),
-      warnFn = msg => if (level.value >= NativeLogLevel.Warn.value) out.println(s"[warn] $msg"),
+      debugFn = msg => if (level.value >= NativeLogLevel.Debug.value) err.println(s"[debug] $msg"),
+      infoFn = msg => if (level.value >= NativeLogLevel.Info.value) err.println(s"[info] $msg"),
+      warnFn = msg => if (level.value >= NativeLogLevel.Warn.value) err.println(s"[warn] $msg"),
       errorFn = msg => if (level.value >= NativeLogLevel.Error.value) err.println(s"[error] $msg")
     )